Various techniques have been employed for transferring or transporting thermal energy between a thermal source and a thermal sink or load. In some instances, there is relatively little concern for the amount of heat which may be lost, as by radiation, convection or conduction during the transport process, either because the transport distance is small and the loss is slight or because of an excessive amount of thermal energy available for transport. In other instances however, it is desired to transfer the heat as efficiently as possible and heat pipes have been especially useful in accomplishing that end. A heat pipe is generally considered a closed-loop two cycle system. In a vaporization/condensation type of heat pipe, there is a rapid heat transfer into the pipe resulting in evaporation of a working fluid therein. The evaporated working fluid builds up sufficient pressure to be transmitted along the pipe and is then condensed at the other end thereof. The cycle is completed by returning the condensate to the evaporating end by means of capillary or other action through a wick or other suitable means within the pipe. Typically, the working fluid may be water, freon, methyl alcohol, acetone or the like. However, because the working fluid is at an elevated-temperature vaporized state while transporting thermal energy from the heat source to the heat sink, it may experience significant radiation, convection and conduction losses to the environment which is normally at a significantly lower temperature. These losses may be considerable if the transport path length is particularly long and therefore such vaporization/condensation heat pipes are generally used only where the distance over which the heat is to be transported is relatively short or insignificant, as for instance less than ten feet.
In many instances, however, it is desired or required to transport thermal energy over relatively long or significant distances from tens or hundreds of feet to as much as tens or hundreds of miles. To accomplish such transport in a relatively efficient manner, resort has been made to chemical heat pipe systems in which a reactant or reactants within the system undergo a first chemical reaction at the heat source and a second chemical reaction at the heat sink. That first chemical reaction is of an endothermic nature in which heat is chemically absorbed by the reaction process and the second reaction is an exothermic reaction during which heat is chemically liberated during the reaction process. Importantly, the reactant and/or reaction products may exist and be transported at temperatures which do not differ substantially from that of the environment, thereby greatly reducing the potential for thermal loss from the system. These chemical reactions are generally reversible and are generally effected and/or accelerated by a catalyst at one or both of the reaction sites.
Examples of such chemical heat pipes, especially for the delivery of thermal energy over long distances as from a nuclear reactor to remote industrial and/or residential heat sinks, are disclosed in the following U.S. Pat. Nos. 3,198,710 issued Aug. 3, 1965 to R. B. Long; 3,558,047 issued Jan. 26, 1971 to Wolfgang et al; 3,690,550 Sept. 12, 1972 to Hilberath et al; 3,967,676 issued July 6, 1976 to H. S. Spacil; 4,044,821 issued Aug. 30, 1977 to J. C. Fletcher; 4,091,864 issued May 30, 1978 to Cocuzza et al; and 4,169,499 issued Oct. 2, 1979 to LeFrois. The U.S. Pat. Nos. 4,044,821 and 4,169,499 are examples of systems in which thermal energy from a source at one temperature is transported for use at a substantially higher temperature at the thermal sink. The remainder of the above cited U.S. patents generally transfer thermal energy from a high temperature heat source to a relatively lower temperature heat sink. A variety of reactants have been used in those systems including cyclohexane and/or possibly methylcyclohexane in U.S. Pat. Nos. 3,198,710 and 4,044,821, phosgene in U.S. Pat. No. 3,967,676, urea in U.S. Pat. No. 4,169,499, sulphur trioxide in U.S. Pat. No. 4,091,864 and ethane and/or methane in U.S. Pat. Nos. 3,558,047 and 3,690,550. The systems disclosed in the aforementioned patents normally provide various supplemental means for transporting the reactant or reactants and the products of reaction between the heat source and heat sink. Typically, one or more pumps and/or blowers are provided in the system's flow path for effecting the requisite reactant transport. These driving or transport mechanisms represent a cost in the system in terms of the equipment itself and/or the energy consumed to drive the pumps. This cost may be considerable if the system is of large capacity and/or is to transport the reactant for significant distances, and in some instances may be such as to negate the advantages otherwise afforded by such a system.
Accordingly, it is a principal object of the present invention to provide a method and apparatus which efficiently transports thermal energy for significant distances. Included in this object is the provision of an improved chemical heat pipe.
It is the further object of the present invention to provide a chemical heat pipe which effects transport of the working medium without inclusion of a conventional active driving mechanism. Included within this object is the provision of a self-driven chemical heat pipe which utilizes the properties of the reactant and reaction products and system geometry to effect a requisite flow of the working medium.
It is a still further object to provide a chemical heat pipe in accordance with the invention which is capable of selectively storing and releasing heat respectively to and from storage.
In accordance with the present invention, there is provided a method and apparatus for the efficient transport of thermal energy, particularly for relatively long distances in a self-driven system employing a chemical heat pipe. The heat pipe employs reversible endothermic and exothermic chemical reactions at respectively spaced heat source and heat sink positions therein for chemically transferring heat from a heat source to a heat sink. The chemical reactant or reactants provide the reversible endothermic/exothermic reactions, particularly in the presence of a suitable catalyst. A preferred type of reaction involves the dehydrogenation and hydrogenation of the reactant and the reaction products respectively, a preferred example of one such reactant being methylcyclohexane.
Importantly, the transport of the reactant and the reaction products which comprise the working medium of the heat pipe, is accomplished without requiring the insertion of conventional pumps into the heat pipe system. Instead, the chemistry of the reaction process and a selective valve technique within the heat pipe system operate to establish a substantially unidirectional flow of the reactant and reaction products within the closed circuit of the heat pipe system, which flow is operative to transfer relatively large quantities of thermal energy over significant distances. Moreover, operation of the heat pipe results in relatively little or no loss in the thermal energy transferred.
In a preferred embodiment, at least a portion of the flow conduit is occluded between the heat sink and the heat source positions with a liquid medium through which the reactant may flow but which effectively blocks the reverse flow of reaction products. The liquid medium is preferably caused to flow toward the heat source position, as by gravity, and fills a U-trap or the like, preferably near the heat source position. The liquid medium may preferably be the condensate of the reactant following its recombination from the reaction products at the heat sink. External cooling coils may effect the requisite condensation. Alternatively, the liquid medium may comprise a liquid solvent into which the recombined reactant is dissolved and through which that reactant is capable of passing to the heat source. The reactant in or from the liquid medium is delivered to the heat source position at which it undergoes vaporization and subsequent endothermic reaction in the presence of a catalyst.
In a further embodiment of the invention, the endothermic reaction product, normally in a gaseous form, is selectively diverted to a long-term storage device prior to arriving at the heat sink if no heat is required at the latter and is selectively retrievable from that storage for use at the heat sink when heat is needed thereat and is unavailable from the heat source. The reaction product constituent or constituents may be condensed and stored in liquid form and/or absorbed, as by adsorption, and stored. The process may be reversed to return the reaction products to the system for reaction at the heat sink.